An oscillator is an electronic device that can produce a periodic, oscillating electronic signal. A circuit with an oscillator is called an oscillator circuit. There are many types of oscillators. For example, categorized by the method of excitation, there are self-excited oscillators and separately excited oscillators. Categorized by the circuit structure, there are RC (resistor-capacitor) oscillators, LC (resistor-inductor) oscillators, crystal oscillators, and ring oscillators, and tuning fork oscillators. Further, categorized by the output waveform, there are sine-wave oscillators, square-wave oscillators, sawtooth oscillators. Oscillator circuits are commonly used in devices for telecommunication, broadcasting, television, etc. Widely used oscillator circuits include RC oscillator circuits, ring oscillator circuits, and crystal oscillator circuits.
RC oscillator circuits are among the most popular oscillator circuits. An RC oscillator circuit has a simple circuit structure with low production cost and low power consumption. However, performance of an RC oscillator circuit is often closely correlated to the device parameters. The output frequency of an RC oscillator circuit can be significantly affected by the working voltage. Thus, the output frequency of an RC oscillator circuit can sometimes have low accuracy. Ring oscillators have low production cost and the frequency range of the output signal can be considerably wide. However, a ring oscillator circuit can be very sensitive to noises from power supply, and the output frequency of a ring oscillator can also have low accuracy. Crystal oscillator circuits are stably operated, and the accuracy of the output frequency of a crystal oscillator circuit is only determined by the natural frequency of the selected crystal device, so the output frequency normally has a higher accuracy. However, a crystal oscillator circuit often takes up a considerably large area and is not able to be integrated within a chip. In addition, when a quartz crystal is used, production cost can be high.
As can be seen, among the three most commonly used oscillator circuits, RC oscillator circuits and ring oscillator circuits have relatively low production cost, but have low output frequency accuracy. Crystal oscillator circuits have high output frequency accuracy, but also have high production cost. Therefore, a low-cost oscillator circuit with high output frequency accuracy is desired.